Medical thermometer having an improved optics system

ABSTRACT

A medical thermometer including a curved mirror and a radiation sensor is disclosed. The radiation sensor is disposed relative to the mirror in a configuration whereby the mirror reflects away from the sensor radiation that passes through the radiation entrance and that is oriented outside a range of angles relative to the mirror, and reflects toward the sensor radiation that passes through the radiation entrance and that is oriented within a range of angles relative to the mirror.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. provisional application No.61/728,003, filed Nov. 19, 2012, the disclosure of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relate generally to devices for measuringtemperature, and more specifically to non-contact infrared thermometersfor medical applications incorporating mirrors to reduce the effects ofstray radiation.

DESCRIPTION OF RELATED ART

A thermal radiation or infrared (IR) thermometer is a device capable ofmeasuring temperature without physically contacting the object ofmeasurement. Thus, such thermometers are often called “non-contact” or“remote” thermometers. In an IR thermometer, the temperature of anobject is taken by detecting an intensity of the IR radiation that isnaturally emanated from the object's surface. For objects between about0° C. and 100° C., this requires the use of IR sensors for detectingradiation having wavelengths from approximately 3 to 40 micrometers.Typically, IR radiation in this range is referred to as thermalradiation.

One example of an IR thermometer is an “instant ear” medicalthermometer, which is capable of making non-contact temperaturemeasurements of the tympanic membrane and surrounding tissues of the earcanal of a human or animal. Instant ear thermometers are exemplified byU.S. Pat. No. 4,797,840 to Fraden, which is incorporated by referenceherein in its entirety. Other examples include medical thermometers formeasuring surface skin temperatures (for example, a skin surfacetemperature of the forehead) as exemplified by U.S. Pat. No. 6,789,936to Kraus et al., which is incorporated by reference herein in itsentirety.

In order to measure the surface temperature of an object based on its IRradiation emissions, the IR radiation is detected and converted into anelectrical signal suitable for processing by conventional electroniccircuits. The task of detecting the IR radiation is accomplished by anIR sensor or detector.

Conventional thermal IR sensors typically include a housing with aninfrared transparent window, or filter, and at least one sensing elementthat is responsive to a thermal radiation energy flux Φ emanating froman object's surface that passes through the IR window of the IR sensorand onto the sensing element. The IR sensor functions to generate anelectric signal, which is representative of the net IR flux Φ existingbetween the sensing element and the object of measurement. Theelectrical signal can be related to the object's temperature byappropriate data processing, as is known in the art.

Thermal flux Φ is a function of two temperatures: a sensing elementsurface temperature T_(s) and a surface temperature of the object T_(b)(measured in Kelvin). Theoretically, Planck's law describes the amountof electromagnetic energy with a certain wavelength radiated by a blackbody in thermal equilibrium. For a broad optical spectral range, whichmay be determined by an optical system of the IR thermometer, therelationship between the two temperatures T_(s), T_(b) and the flux Φmay be approximated by a fourth-order parabola. This approximation isknown as the Stefan-Boltzmann law:

Φ=κε_(b)ε_(s)σ(T _(b) ⁴ −T _(s) ⁴)   (1)

where ε_(b) and ε_(s) are the surface emissivities of the object andsensing element, respectively, σ is the Stefan-Boltzmann constant, and κis an optical constant which may be determined by measurement duringcalibration of the IR thermometer.

For a relatively small difference between the object's true temperatureT_(b) and sensor's temperature T_(s), Eq. (1) can be approximated as:

Φ≈4κε_(b)ε_(s)σT_(s) ³(T_(b)−T_(s))   (2)

An objective of the IR thermometer is to determine the surfacetemperature of the object, T_(b), which may be calculated as T_(bc) frominverted Eq. 2:

$\begin{matrix}{T_{bc} = {T_{s} + \frac{\Phi}{4{\kappa ɛ}_{b}ɛ_{s}\sigma \; T_{s}^{3}}}} & (3)\end{matrix}$

Ideally, the computed temperature T_(bc) should be equal to the truetemperature T_(b). Practically, these temperatures may differ as theresult of, e.g., measurement error or calibration drift. It can be seenfrom Equation (3) that, in order to calculate temperature T_(bc), twovalues need to be determined: the magnitude of the IR flux Φ and the IRsensing element's surface temperature T_(s). The accuracy of thetemperature computation depends on the measurement accuracy for allvariables on the right side of Eq. (3). The first summand T_(s) can bemeasured quite accurately by a number of techniques known in the art,for example, by employing a thermistor or RTD temperature sensor. Thesecond summand can be more problematic, especially due to a generallyunknown and unpredictable value of the object's emissivity ε_(b). Forexample, in medical thermometry, the emissivity ε_(b) is a skinemissivity that is defined by the skin properties and shape. The skinemissivity may, for example, range from 0.93 to 0.99.

To determine how emissivity affects accuracy, a partial derivative ofEq. (2) may be calculated as:

$\begin{matrix}{\frac{\partial\Phi}{\partial ɛ_{b}} = {4{\kappa ɛ}_{s}\sigma \; {T_{s}^{3}\left( {T_{b} - T_{s}} \right)}}} & (4)\end{matrix}$

The partial derivative represents the measurement error due to anunknown emissivity ε_(b) of an object. Eq. (4) shows that the errorapproaches zero as T_(s) approaches T_(b). Accordingly, when T_(b)approximately equals T_(s), the error is small. Thus, to minimizeerrors, it is desirable to keep the temperature T_(s) of the IR sensoras close as is practical to the object's temperature T_(b). For aninstant ear thermometer, for example, U.S. Pat. No. 5,645,349 to Fraden,incorporated by reference in its entirety herein, teaches a heatedsensing element for bringing the temperatures T_(s) and T_(b) intoproximity of each other. U.S. Pat. No. 7,014,358 to Kraus et al.,incorporated by reference in its entirety herein, alternatively teachesa heating element for warming the IR sensor housing. Additionally, U.S.Patent Application Publication No. U.S. 2011/0228811 to Fraden,incorporated by reference in its entirety herein, teaches shielding thesensor from stray radiation using a shield that is also heated totemperature T_(b).

When temperature is measured from a surface, it is important to minimizethe amount of radiation received at the IR sensor that emanated fromunwanted sources. One way to minimize the chance of picking up unwantedor stray radiation is to narrow the optical field of view of the IRthermometer. One method is to use IR lenses to narrow the optical fieldof view as exemplified by U.S. Pat. No. 5,172,978 to Nomura et al.(radiant thermometer including a lens barrel mounting a condensing lensat one end and an IR detector at the other end) and U.S. Pat. No.5,655,838 to Ridley et al. (radiation thermometer with multi-elementfocusing lens, eye piece, beam splitter and IR detector), each of whichis incorporated by reference in its entirety herein.

Another method for minimizing the chance of picking up flux from strayobjects employs mirrors to aid a user of an IR thermometer invisualizing the IR thermometer's field of view. This approach isexemplified by U.S. Pat. No. 4,494,881 to Everest, which is incorporatedby reference in its entirety herein.

While these methods are capable of removing from the sensor's field ofview some of the sources of undesired radiation, it would be ofadditional benefit to remove sources of radiation that are within the IRsensor's field of view, but that emanate from outside of a desiredtarget area within that field of view.

SUMMARY OF THE INVENTION

A non-contact IR thermometer according to various embodiments of thepresent invention includes, among other things, an IR radiation sensorhaving a sensor surface, which may be coupled to a filter positioned inthe sensor's field of view that may be capable of passing only radiationhaving a desired range of wavelengths; a mirror, which may be parabolicor approximately parabolic in shape and may include surfaces andcurvatures based on elliptic paraboloids, the sensor being positioned ator near a focal point of the mirror and the filter being positionedbetween the sensor and the mirror; and an aperture that is outside thesensor's direct field of view, the mirror providing a radiation pathbetween the filter and the aperture. In various embodiments, the sensormay be included as a component on a semiconductor device that possessesvarious additional functionalities as will be understood by those havingordinary skill in the art. Additionally, in various embodiments, thecenter of the sensor surface may be positioned at or near the focalpoint of the mirror and the surface of the sensor may be oriented atvarious angles with respect to the baseline of the mirror to furtherminimize the amount of stray radiation reaching the sensor, which may bedetermined or understood as a percentage of total radiation. In variousembodiments, the angle between the baseline of the mirror and the normalto the surface of the mirror is between approximately 25° and 35°. Inother embodiments, this angle is approximately 31.5°. In variousembodiments the aperture may include, be covered by, or have disposedadjacent thereto a protective window and/or filter that can preventradiation of certain undesired wavelengths from passing therethrough.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will be morereadily apparent from the following detailed description and drawings ofillustrative embodiments of the invention in which:

FIG. 1 is a cross-sectional view of an IR thermometer according to anembodiment of the present invention;

FIG. 2 is a cross-sectional view of an IR thermometer according to anembodiment of the present invention;

FIG. 3 is a cross-sectional view of an IR thermometer according to anembodiment of the present invention;

FIG. 4 is a cross-sectional view of an IR thermometer according to anembodiment of the present invention;

FIG. 5 is a cross-sectional view of an IR thermometer according to anembodiment of the present invention;

FIG. 6 is a cross-sectional view of an IR thermometer according to anembodiment of the present invention; and

FIG. 7 is a cross-sectional view of an IR thermometer according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A remote IR thermometer is disclosed that includes, among other things,a parabolic or approximately parabolic mirror and an IR radiation sensorassembly including a filter component and a sensor component. The sensorcomponent includes a surface with a geometric center point on thesurface that is positioned in the vicinity of the mirror's focal point.The sensor component may be oriented about the center point at variousangles. For the purpose of illustrating principles in accordance withvarious embodiments of the present invention, several non-limitingexamples of the various embodiments are described below. Accordingly,the scope of the invention should be understood to be defined only bythe scope of the claims and their equivalents, and not limited by theexample embodiments.

FIG. 1 shows a schematic, cross-sectional view of an embodiment of themirror 20 and sensor assembly 30 inside a remote IR thermometer 10having a radiation entrance, e.g., aperture 16 that may include, becovered by, or have disposed adjacent thereto a protective window and/orfilter 55. Mirror 20 may be parabolic or approximately parabolic inshape so as to define a focal point 50 near to or along the axis ofsymmetry 52, as defined by the mirror's parabolic or approximatelyparabolic curvature 58, which is perpendicular to the mirror's baseline54, the baseline being a line tangent to the mirror at the base orvertex of the mirror (or the parabolic or approximately parabolic shapethereof). The general equation for a parabola is y=ax²+bx+c, where a andb are constants that define the shape of the parabola and c is aconstant that defines the position of the parabola's vertex with respectto an origin. In various embodiments, a may be approximately between,e.g., 0.01 and 2.0, or approximately between 0.07 and 0.09, and moreparticularly, approximately 0.5, 0.08, or 0.0799. In various embodimentsb may be approximately between, e.g., −2.0 and 2.0, or approximatelybetween −0.02 and −0.01, and more particularly approximately 1.0, −0.02,or −0.015. Because the definition of c is relative to an origin, andbecause c does not affect the shape of the parabola, a person havingordinary skill in the art will appreciate that c does not need to bedefined to carry out the various embodiments of the invention disclosedherein. In various embodiments, a and b are chosen such that thecorresponding focal point may be located on the axis of symmetry, atvarious positions above the corresponding vertex. In variousembodiments, axis of symmetry 52 is nominally perpendicular to aperture16. In various embodiments, axis of symmetry 52 may pass through a lowerportion of aperture 16. In other embodiments, axis of symmetry 52 maypass below aperture 16. In various embodiments, the mirror surface isdefined by sweeping or rotating any of the parabolas heretoforedescribed about the axis of symmetry 52. In other embodiments the mirrormay also include curvatures and surfaces that may be described by theequation for an elliptic paraboloid, i.e.,

${\frac{z}{g} = {\frac{x^{2}}{d^{2}} + \frac{y^{2}}{f^{2}}}},$

where d and f are constants that dictate the degree of curvature in thex/z and the y/z planes, and g is a scaling constant.

Sensor assembly 30 includes at least a sensor component 32 that includesa detection surface 42 with a geometric center point 44 thereon that ispositioned in the vicinity of the mirror's focal point 50. As shown inFIG. 1, center point 44 is disposed at focal point 50. Surface 42 may beoriented at various angles a (formed between the normal to surface 42and baseline 54 of the mirror) so that surface 42 faces at least aportion of mirror 20. In various embodiments sensor assembly 30 may alsoinclude a filter component 40 adjacent to or abutting sensor component32. When a sensor assembly 30 including a filter component 40 is used inIR thermometer 10, filter component 40 may be disposed between sensorcomponent 32 and mirror 20.

In various embodiments, mirror 20 is disposed inside thermometer 10 suchthat aperture 16 is in the line of sight of mirror 20. So disposed,mirror 20 may reflect radiation toward sensor assembly 30 that wasemitted from a portion of an object 14 in the field of view of aperture16 and passed through aperture 16 and protective window and/or filter55.

The amount of radiation incident upon mirror 20 that is directed ontosurface 42, i.e., that the sensor can detect, is a function of the angleα. In various embodiments, including those embodiments where the mirrorhas parabolic shapes, curvatures, or surfaces, surface 42 may beoriented so that α is between approximately 25° and approximately 35°.In various embodiments, e.g., where the mirror has a parabolic shapedefined by a being approximately 0.0799 and b being approximately−0.015, α may be set at approximately 31.5°. For these embodiments,sensor component 32 primarily receives radiation that approaches mirror20 at a angles of less than approximately five degrees above or below aline parallel to axis of symmetry 52. Such a range of angles may bereferred to as a radiation range of angles. Conversely, sensor component32 receives only a minimal or negligible portion of the radiation thatapproaches mirror 20 at a radiation range of angles greater thanapproximately six degrees above or below a line parallel to the axis ofsymmetry 52 because, given the mirror's shape and the size of surface42, radiation oriented at these larger angles is not reflected by themirror along a path that intersects with or reaches surface 42. Forillustration, FIG. 2 depicts radiation that is directed toward mirror 20in a direction parallel to axis of symmetry 52. The mirror reflects mostor all of this radiation, which then passes through filter component 40to strike sensor surface 42 near to center point 44, FIG. 3 depictsradiation that is directed approximately five degrees above a lineparallel to axis of symmetry 52. The mirror reflects this radiation,which then passes through filter component 40 to strike surface 42 nearto the right edge of sensor component 32. FIG. 4 depicts radiation thatis directed approximately five degrees below a line parallel to axis ofsymmetry 52. The mirror reflects this radiation, which then passesthrough filter component 40 to strike sensor surface 42 near to the leftedge of sensor component 32. FIG. 5 depicts radiation that is directedapproximately six degrees above a line parallel to axis of symmetry 52,and FIG. 6 depicts radiation that is directed approximately six degreesbelow a line parallel to axis of symmetry 52. In these latter two cases,the mirror reflects the radiation, which then passes through filtercomponent 40; however, the reflected radiation does not strike sensorcomponent 32, falling too far to the right (FIG. 5) or too far to theleft (FIG. 6). FIG. 7 depicts radiation that is directed approximately12 degrees below a line parallel to axis of symmetry 52, which moreclearly show that the reflected radiation does not strike sensorcomponent 32. Accordingly, by selectively positioning the mirror inthese and other embodiments, undesired radiation that does not emanatefrom a portion of a surface disposed in front of aperture 16, such thatthis radiation is oriented outside of a desired radiation range ofangles, may be diverted away from sensor component 32. Correspondingly,sensor component 32 does not detect this undesired radiation. However,sensor component 32 can detect desired radiation emanating from aportion of a surface disposed in front of aperture 16 because thisradiation is oriented inside the desired radiation range of angles andreaches sensor component 32. In this way, stray radiation emanating fromobjects other than the intended object can be prevented from reachingthe sensor and being detected.

In various embodiments, filter component 40 may be an infrared band-passtype filter made of silicon that allows radiation having wavelengthsbetween approximately, e.g., 7.5 μm and 13.5 μm to reach surface 44.Such a filter prevents, e.g., visible light and far infrared light fromreaching the sensor and affecting the sensor's output. Additionally,such a filter may be used to reduce the intensity of the radiation inthe range of desired wavelengths, e.g., IR radiation, that reaches thesensor, which may improve the accuracy and the repeatability of thesensor. In certain embodiments, the intensity of the radiation passingthe filter and reaching the sensor is one-seventh of the radiation thatreflects from the mirror and reaches the filter. A non-limiting exampleof a sensor that may be used in various embodiments described herein isPart No. TPiS 1T 1252, manufactured by Excelitas Technologies Corp.

While the various embodiments of the invention have been particularlyshown and described, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the invention. Accordingly, theseembodiments are non-limiting examples of the invention and the inventionshould be understood to be defined only by the scope of the claims andtheir equivalents.

1. A medical thermometer, comprising: a housing having a radiationentrance; a mirror disposed in the housing, a reflective surface of themirror having a parabolic shape defining a vertex, a baseline tangent tothe parabolic shape at the vertex, an axis of symmetry perpendicular toboth the baseline and to the radiation entrance, and a focal point; anda radiation sensor having a detection surface, the radiation sensordisposed in the housing at the focal point such that a normal to thedetection surface is angled from 25° to 35° with respect to the baselineof the reflective surface, wherein, the radiation entrance, the mirrorand the radiation sensor are configured for the detection surface toreceive radiation passing through the radiation entrance from 5° to −5°of a line parallel to the axis of symmetry, and for the mirror toreflect away from the detection surface radiation passing through theradiation entrance not within 6° and −6° of a line parallel to the axisof symmetry.
 2. The medical thermometer of claim 1 wherein the parabolicshape is defined by the equation y=ax²+bx+c, where a is not equal to 0.3. The medical thermometer of claim 2 wherein a is from 0.01 to 2.0. 4.The medical thermometer of claim 2 wherein b is from −2.0 to 2.0.
 5. Themedical thermometer of claim 1 wherein the detection surface has acenter point thereon and the center point is disposed approximately atthe focal point.
 6. The medical thermometer of claim 2 wherein a is from0.07 to 0.09.
 7. The medical thermometer of claim 6 wherein b is from−0.02 to −0.01.
 8. The medical thermometer of claim 7 wherein thedetection surface has a center point thereon and the center point isdisposed approximately at the focal point.
 9. The medical thermometer ofclaim 8 wherein the normal to the detection surface is angled from 31°to 32° with respect to the baseline.
 10. The medical thermometer ofclaim 1, further comprising a filter positioned in the field of view ofthe radiation sensor and between the mirror and the radiation sensor,the filter being capable of passing only radiation having apredetermined range of wavelengths.
 11. The medical thermometer of claim10, wherein the filter is capable of passing only radiation havingwavelengths from 7.5 μm to 13.5 μm.
 12. The medical thermometer of claim10, wherein the filter is a component of a radiation sensor assemblywhich includes the radiation sensor.
 13. A medical thermometer,comprising: a housing having a radiation entrance; a parabolic mirrorhaving a vertex, a focal region, a baseline tangent to the mirror at thevertex and an axis of symmetry perpendicular to both the baseline and tothe radiation entrance; and a radiation sensor having a center point ona detection surface, the center point being disposed at leastapproximately at the focal region, and a normal to the detection surfacebeing oriented at a detection angle with respect to the baseline, thedetection angle being from 25° to 35°, wherein, the radiation entrance,the mirror and the radiation sensor are configured for the detectionsurface to receive radiation passing through the radiation entrance from5° to −5° of a line parallel to the axis of symmetry, and for the mirrorto reflect away from the detection surface radiation passing through theradiation entrance not within 6° and −6° of a line parallel to the axisof symmetry.
 14. The medical thermometer of claim 13 wherein theparabolic mirror has a shape defined by the equation y=ax²+bx+c, where ais not equal to 0, and further wherein the focal region is a focalpoint, and the parabolic shape a parabola vertex coincident with thevertex of the mirror.
 15. The medical thermometer of claim 14 wherein ais from 0.01 to 2.0.
 16. The medical thermometer of claim 14 wherein bis from −2.0 to 2.0.
 17. The medical thermometer of claim 14 wherein ais from 0.07 to 0.09.
 18. The medical thermometer of claim 17 wherein bis from −0.02 to −0.01.
 19. The medical thermometer of claim 18 whereinthe detection angle is from 31° to 32°.
 20. The medical thermometer ofclaim 13, further comprising a filter positioned in the field of view ofthe radiation sensor and between the mirror and the radiation sensor,the filter being capable of passing only radiation having apredetermined range of wavelengths.
 21. The medical thermometer of claim20, wherein the filter is capable of passing only radiation havingwavelengths from 7.5 μm to 13.5 μm.
 22. The medical thermometer of claim20, wherein the filter is a component of a radiation sensor assemblywhich includes the radiation sensor.
 23. A medical thermometer,comprising: a housing having a radiation entrance; a parabolic mirrordefined by the equation y=ax²+bx+c, where a is from 0.07 to 0.09, b isfrom −0.02 and −0.01, the parabolic mirror having an axis of symmetry, avertex, a baseline tangent to the mirror at the vertex, the axis ofsymmetry perpendicular to both the baseline and to the radiationentrance, and a focal point; and a radiation sensor having a centerpoint on a detection surface, the center point being disposed at thefocal point, and a normal to the detection surface being oriented from30° to 33° with respect to the baseline; wherein, the mirror and theradiation sensor are configured for the detection surface to receiveradiation passing through the radiation entrance from 5° to −5° of aline parallel to the axis of symmetry, and for the mirror to reflectaway from the detection surface radiation passing through the radiationentrance not within 6° and −6° of a line parallel to the axis ofsymmetry,
 24. A method of using a medical thermometer including ahousing having a radiation entrance, a parabolic mirror disposed in thehousing, the parabolic mirror having a vertex, a baseline tangent to themirror at the vertex, and an axis of symmetry perpendicular to both thebaseline and to the radiation entrance, and a radiation senor having adetection surface, a normal to the detection surface being orientedbetween 30° and 33° with respect to the baseline, comprising; disposingthe thermometer relative to a target; and directing the radiationentrance toward the target, receiving a temperature value determinedfrom radiation that passes through the radiation entrance from 5° to −5°of a line parallel to the axis of symmetry received by the detectionsurface, while radiation that passes through the radiation entrance notwithin 6° and −6° of the line parallel to the axis is reflected awayfrom the detection surface by the mirror.
 25. The method of claim 24wherein the mirror has a parabolic shape as defined by the equationy=ax²+bx+c, where a is not equal to 0, the parabolic shape having afocal point, a vertex, and an axis of symmetry collinear with the axisof the mirror.
 26. The method of claim 25 wherein a is from 0.07 to0.09.
 27. The method of claim 26 wherein b is from −0.02 to −0.01. 28.The method of claim 24 wherein the detection surface has a center pointthereon and wherein the center point is disposed approximately at thefocal point.